Ionic Current and Film Growth of Thin Oxide Layers on Aluminium

This paper describes part of a series of investigations into the electrical properties of thin insulating oxide layers formed electrolytically on aluminium, and is limited to a study of the ionic current and the resultant film growth. It is shown that at a given temperature the ion current i is a function of the average electrostatic field strength F across the film. Two forms of the function i = f ( F ) previously proposed are considered. They have been investigated in three ways: (i) ion current as a function of applied voltage V f at constant film thickness δ, (ii) current decay with time during formation at constant formation voltage, (iii) change of film thickness or capacity with time at constant formation voltage. These results tend to favour the exponential relationship i = A exp BF with A = 10 -18 A/cm 2 and B = 3 × 10 -6 cm/v at room temperature. The law of film growth derived is only approximately logarithmic, whilst during current decay i similar 6 × 10 -5 V f / t . At higher temperatures it is shown that the approximately logarithmic law is replaced by a cubic law over a short range of temperatures, and subsequently by a quadratic law. The application of these results to oxidation in air is discussed.